Hepatitis C virus (HCV) is a leading cause of chronic hepatitis and hepatocellular carcinoma worldwide and infects more than 1% of the world?s population. Successful vaccine development is pivotal in controlling this global health problem. A system for efficient assembly of HCV structural proteins into HCV-like particles (VLPs) in insect cells has been developed in our laboratory. These noninfectious HCV-like particles had similar morphologic, serologic and biophysical properties as the putative virions isolated from HCV infected humans. In contrast to recombinant subunit vaccines, the viral proteins of HCV-like particles may be presented in a native, virion-like conformation and may therefore be superior in eliciting a protective humoral and cellular immune response. Using HCV-LP as a capture antigen in an enzyme-linked immunosorbant assay (ELISA), anti-HCV antibodies were specifically detected in serum from individuals with acute and chronic HCV infection. The ability of the HCV-LP to elicit an immune response in vivo was studied in mice. Anticore and antienvelope antibodies were detected in all HCV-LP immunized mice with antienvelope titers ranging from 51,200 to 204,800. The induced antienvelope antibodies recognize E2 proteins of different genotypes and were broadly directed against all three domains of the E2 protein. Splenocytes from HCV-LP immunized mice showed T cell proliferative responses against either core or E1/E2 proteins. Cytotoxic T lymphocyte activities were also assayed against syngeneic P815 cells expressing HCV structural proteins together or individually. Splenocytes from HCV-LP immunized mice showed an HCV-specific cytolysis that was mostly directed against E2. In addition, IFN-g but not IL-4 production could be detected in activated HCV-specific T cells, possible suggesting a type 1-like response against HCV-LP. We are conducting experiments in chimpanzees to test the effectiveness of the HCV-LP as a vaccine candidate. Our studies demonstrate that HCV-LP are capable of inducing a humoral and cellular immune response targeted against various regions of HCV structural proteins and may be promising as a potential vaccine candidate.